One of the reasons is segregation of Mn in the inter-dendritic region. During hot rolling, the Mn enriched region reduces the activity of carbon and gives rise to more pearlite, in contrast to enriched ferrite in Mn depleted regions.
YOU HAVE MORE THAN Mn AND CARBON THERE. YOU NEED TO FIND THE IMPURITIES IN THE STEEL AND WHAT SEGREGATES AT THE GRAIN BOUNBDARIES. TYPE OF THE IMPURITIES AND GRAIN SIZE PLAY MAJOR ROLE.
Bands in ferrite/pearlite microstructures are related to the microsegregation of alloying elements during solidification. Elements like Mn or Si are rejected from delta ferrite dendrites, which causes interdendritic regions with high Mn or Si concentrations. This is retained during transformation from austenite to ferrite and provides basis for microstructural banding.
Please see the following paper or links: might be helpful.
1) Acta Metallurgica Sinica (Enghish Letters), 2005 , Vol.18, No. 5, pp.664-668, by C.L. Mo et al: "The micro-structural banding in the centre of hot rolling strip".
2) ASM Metals Reference Book, 3rd Edition by Michael Bauccio, ASM International, 01-Jan-1993 - Technology & Engineering.
3) "Suppression of banded structure and refinement of the microstructure of high-strength cold-rolled steels by optimisation of the hot-rolling cooling path": please see the attached file that i downloaded from the Google search.
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